In article ,
Larry W4CSC wrote:
"TimJay" wrote in news:wdidnd9mKMdB5y_fRVn-
:
Need info about the Open Array 72" verse 48"
In any radar system, from any manufacturer, you need to think about how far
it is to the horizon from the altitude your antenna is located. Ask
yourself, "Can this system give me good returns from a normal boat at the
horizon?" It cannot see beyond its horizon. Its horizon is dependent on
its altitude. Altitude is also a tradeoff with close-in target acquistion.
If you put the antenna up high to get a longer horizon, you won't be able
to see that damned bouy someone put in your way in the fog you are about to
run over. I'll trade seeing that bouy in really close to the boat for
seeing a boat out 35 miles any day of the week. Of course, having two
radars, one low down to see that bouy and one up high to see those long-
range targets is much nicer. It also gives you redundancy for the
inevitable failures of these cheaply made boat systems.
A 72" array WILL have a narrower beamwidth than the 48". A narrower
beamwidth translates into resolution, not range, of the targets on the
scope. Instead of the two targets out there painting as one wide one, you
MAY be able to see them as two distinct targets, as if that really made any
difference. AS you get closer to the targets, the wider beamwidth antenna
also shows them as separate targets closer in, which I think makes the
point moot. The higher power of the 72" transmitter WILL show small
targets like towers and bouys at larger ranges by painting them with more
brute RF power force, creating a larger return signal above the noise floor
of the receiver. If seeing a bouy 18 miles away instead of 10 miles away
is important to you, the higher the power the better. If under sail, with
limited power resources, higher power eating the batteries soon becomes
more important than seeing the bouy so far away. At 30 gallons per hour,
it matters little.
Hope this makes you think of your particular situation. Everything is a
compromise.
Open Arrays are really SlotLine Waveguide Antennas, and legnth is
proportional to Antenna Gain, and Transmitted Peak Power. It is also
Inversely proportional to Horozontal Beamwidth. More is better, for
all cases, and less beamwidth is better always. I only have one nit to
pick with Larry's above, and that is Detection Distance for any
particular target is a combination of all the above factors and a few
more, but one of the biggies is Peak Transmitted Power, More power,
equals better targets, at longer ranges. In the old days before Loran,
and GPS, Radar was used for Position Fixing, and mountains were often
used as targets for navigation, at ranges exceeding 72 miles, with the
Decca 3xx, and 4xx series radars that had 20Kw and 40Kw XBand
transmitters with 6 and 7.5 Ft antennas.
Rule of thumb: Bigger is better, longer is better , but all this
"better" costs alot more money. So, eventually money is the limiting
factor.
Bruce in alaska
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